The serum calcium concentration in mammals is thought to be regulated by a dual feedback system involving the secretion of parathyroid hormone by the parathyroid gland and of calcitonin by the parafollicular cells of the thyroid gland. In vivo and in vitro experiments have demonstrated that the secretion of calcitonin is directly proportional to the calcium concentration of the fluid bathing the glands. However, many other agents also stimulate the secretion of calcitonin, and there is considerable question as to its relative role in the regulation of calcium metabolism, particularly in man. The object of this study is to investigate the factors influencing the secretion and synthesis of calcitonin using an in vitro model in an attempt to clarify the mechanisms involved and the physiological importance of calcitonin. Thyroid glands obtained from fetal rats and ultimobranchial glands obtained from embryonic chicks will be cultured in a chemically defined medium. CT activity in this media will be assayed in vitro by measuring the inhibition of parathyroid hormone stimulated release of previously incorporated Ca45 from cultured fetal rat bone shafts.